1. Field of the Invention
The present invention relates generally to high modulus fibers and methods for their handling and storage. More particularly, the present invention involves the design and production of a protective coating system for high modulus fibers used in manufacturing high strength woven fabric reinforcements.
2. Description of Related Art
During the past several years many researchers in the structural materials and sports equipment industries have focused on developing low density materials having increased strength and higher temperature resistance. For example, low weight structural materials having high stiffness and strength at temperatures in excess of 3,000.degree. F. are in high demand for advanced aerospace applications. Similarly, low density, high impact resistant materials are continually in demand for fabricating racquets and related equipment used in sporting events.
In particular, continuous reinforced composite materials or ceramic matrix composites have enjoyed increased use as high temperature, high strength structural materials. These materials can be prepared by impregnating a woven fabric reinforcement, or prepreg, with a ceramic slurry, then subjecting the impregnated fabric to conventional lay-up procedures to form a molded lay-up. After heat treating or firing the molded lay-up, a relatively low density, high strength ceramic object forms which is useful in a number of high temperature structural applications.
Generally, the woven fabric reinforcement is formed of continuous fibers such as silicon carbide, alumina, mullite, graphite, or quartz, which are woven like a yarn in conventional weaving equipment to form the fabric. These materials can have a relatively high modulus, which makes them generally stiff, brittle, and prone to breaking when placed in a bent configuration. Fortunately, many of these high modulus materials can be drawn or made into such small diameter fibers or filaments that they can be bent and even tied into knots without breaking. Thus, fabric reinforcements formed of very fine diameter, high modulus continuous fibers can be handled and woven without breaking the fibers. Additionally, lay-up procedures can be performed using these woven fabrics without risking a decrease in the fabric's structural integrity due to fiber breakage.
More recently, materials technology has led to the development of boron, yttrium, yttrium aluminum garnet, and single crystal alumina (sapphire) and mullite continuous fibers. In particular, single crystal alumina fibers have remarkably high strengths and elevated temperature service properties, making them especially attractive for woven fabrics used for the manufacture of continuous fiber ceramic matrix composites. A major problem associated with handling sapphire fibers and producing viable continuous sapphire fiber woven fabrics is the limited fiber diameter available using fiber drawing procedures. Current production techniques for drawing single crystal alumina fibers are limited to drawing sapphire fibers having a minimum diameter of about 3 mils. Because the modulus of single crystal alumina is on the order of 51.times.10.sup.6 psi, fibers having diameters of 3 mils are not capable of bending less than about a quarter-inch radius without breaking. This bending radius limitation dramatically affects the ability to handle sapphire fibers having lengths suitable for incorporating into continuous fiber fabrics. Thus, handling very high modulus fibers in conventional weaving equipment causes significant fiber breakage, resulting in poor quality and low strength fabrics.
One early approach to this sapphire fiber handling and weaving problem involved chopping continuous length fibers into smaller random lengths ranging from 1 inch to 3 inches and then converting the random lengths into a staple yarn. This technique proved to be unsuccessful because the stiff short sections were uncontrollable during the weaving process. In an attempt to overcome the problem with the staple yarn, the staple process was modified to include serving. This modification resulted in the ability to produce a yarn. However, the yarn had severely limited tensile strength.
In order to address the low tensile strength associated with the staple yarn and serving approach, this method was modified to incorporate a fugitive dacron carrier yarn during the staple yarn forming process and prior to serving. This modification did yield a weavable yarn. However, once the fugitive dacron carrier yarn and the fugitive serving yarn were removed from the woven fabric, subsequent handling caused the yarn to disintegrate and revert to individual fibers in a non-yarn form. This can occur, for example, during a ceramic matrix composite fabric impregnation step.
Still another approach to this handling problem is to develop processes for drawing single crystal alumina fibers having diameters on the order of less than 10 microns which can then be combined to form flexible multifilament weavable yarns. However, because of cost considerations and technical limitations, making small diameter filaments is not viewed as a viable production scale alternative.
Accordingly, it is an object of the present invention to provide processes and associated systems for handling continuous length high modulus fibers in order to reduce or eliminate fiber breakage during handling.
It is a further object of the present invention to provide materials and processes for protecting high modulus fibers during conventional weaving processes in which high strength woven fabrics are produced.